Magnetic recording medium and production processes thereof

ABSTRACT

A magnetic recording medium having an excellent resistance to wear and CSS strength and showing a satisfactory lubricant retention, and processes for production thereof. The magnetic recording medium includes a substrate having formed thereon, in sequence, a magnetic recording layer, a protective coating, and a lubricant coating, the protective coating including a plane portion and a plurality of fine projections and recesses formed on the plane portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a magnetic recording medium, moreparticularly, to a magnetic disc which includes a substrate havingcoated on at least one surface thereof, in sequence, a magneticrecording layer, a protective coating, and a lubricant coating. Themagnetic recording medium of this invention is particularly useful inmagnetic recording devices such as a magnetic disc device based on acontact-start-stop (CSS) system. This invention also relates to aprocess for the production of the magnetic recording medium.

2. Description of the Prior Art

Heretofore, various types of the magnetic recording media have beendeveloped for use in magnetic disc devices for data handling and otherapplications. They generally include a disc-like substrate of aluminum,similar light metals, or their alloys at least one surface of thesubstrate having a magnetic recording layer.

A typical example of the prior art magnetic recording media can be foundin FIG. 1, in which reference number 1 is an aluminum substrate. In theillustrated recording medium, the substrate 1 has an alumite or anodizedaluminum coating 2 effective for enhancing the physical and chemicalproperties of the underlying aluminum substrate and adhesion of amagnetic recording layer 3 to the substrate. There are two types ofmagnetic recording layers: One is a discontinuous magnetic layer, havingdispersed therein binding agents or binders such as synthetic resins,which can be produced by coating a mixture of magnetic powders andbinders on the substrate by means of a spin coater. The other is acontinuous magnetic layer containing no binder. The continuous magneticlayer can be produced, for example, by depositing magnetic materialssuch as iron and iron-cobalt alloys onto the substrate by well-knowntechniques such as sputtering, vacuum evaporation, plating, andelectrodeposition.

Binder-free continuous magnetic recording layers are widely used in theart, since they can be simply produced and have excellent magneticcharacteristics and high recording density. Of the various magneticmaterials, ferrite oxide is particularly useful for the magneticrecording layer because of its hardness and resistance to corrosion.

Magnetic recording media with the above-described continuous magneticlayer, however, are disadvantageous when used in CSS system-basedmagnetic disc devices. In these magnetic disc devices, the magnetic headis generally picked up from and maintained over the surface of themagnetic layer during writing or recording and reading of information.At the start and stop of the rotation of the recording media, however, aslider of the magnetic head contacts and slips on the surface of themagnetic layer. Repeated contact and slippage of the slider with themagnetic layer results in wear on the surface in a head rounding zone ofthe magnetic recording media, spread of fine powders of the wornmagnetic layer over the surface of the recording media, and consequentlycrushing of the magnetic head or briefly "head crush" and destruction ofthe stored information. It is therefore desirable to increase theresistance to wear of the magnetic recording media.

Many useful means and methods have been already proposed to prevent ordecrease wear of the magnetic recording layer during operation of CSSsystem-based magnetic disc devices. One of them is the formation of aprotective coating. For example, Japanese Unexamined Patent Publication(Kokai) No. 52-20804 discloses a magnetic recording medium having aprotective coating of selected polysilicate. Other suitable protectivecoatings are those of synthetic resins or silicon dioxide. The formationof the protective coating is illustrated in FIG. 2. In FIG. 2, the layerconstitution is identical with that of FIG. 1 except that a protectivecoating 4 is further coated as a top layer. This type of protectivecoating, however, cannot completely prevent wear of the underlyingmagnetic layer. The protective coating has a smooth surface due to themirror surface of the alumite coating on the aluminum substrate. Thesmooth surface of the protective coating causes slippage of the magnetichead. This results wears down the coating and again which induces headcrush.

As another means to decrease the wear of the magnetic recording layer,Japanese Unexamined Patent Publication (Kokai) No. 54-161909 suggeststhe use of a combination of a liquid lubricant such asperfluoroalkylpolyether and a solid lubricant such as telomer oftetrafluoroethylene on the magnetic recording layer. These lubricantsmay also be used on the protective coating, if desired. The lubricantcoating formed on the magnetic layer or on the protective coating cansmooth the slippage of the magnetic head for a short time, however, withtime, the lubricant coating becomes thinner and decreases in lubricantaction. As a result, the problems encountered in the previouslydescribed cases also occur. Further, an increase in the amount of thelubricant used would cause undesirable adhesion of the magnetic head tothe surface of the magnetic recording layer. This adhesion of the headmust be avoided, since it results in head crush or destruction of thesupporting means for the magnetic head.

Japanese Unexamined Patent Publication (Kokai) No. 57-20925 teaches theformation of small cylindrical projections on the magnetic layer or theoverlying protective coating. An example of a protective coating havingsmall cylindrical projections can be found in FIG. 3A (cross-sectionalview) and FIG. 3B (perspective view). From these figures, it will beunderstood that a plurality of cylindrical projections 5 having the sameheight are disposed on the protective coating 4. In FIG. 3A, 1 is analuminum substrate, 2 is an alumite coating, and 3 is a magneticrecording layer. In this case, the lubricant is impregnated into thevoids or gaps formed between the adjacent projections. This effectivelyeliminates the problems of adhesion of the magnetic head. However, withtime, the centrifugal force during the rotation of the recording mediumdestroys the uniform distribution of the lubricant. Consequently, thelubricant coating at the outer portion of the recording medium becomesthicker. The lubricant frequently falls from the surface of theprotective coating. It is, therefore, difficult to use the recordingmedium for a long period without the defects described above.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No.56-22221 teaches the formation of small cylindrical recesses on theprotective coating of the magnetic recording medium. This is illustratedin FIGS. 4A and 4B, from which it is apparent that a plurality ofrecesses 6 having the same depth are randomly formed on the protectivecoating 4. The recesses 6 can effectively retain the lubricant, but theycannot prevent the decrease in the resistance to wear of the protectivecoating due to its smooth surface. In addition, the size of the recessesis on the order of 10 μm, since the recesses are produced through amasking process. In practice, recesses having a size on the order of 1μm cannot be produced.

Recently, S. Otaki, one of the inventors of the present case, and fourothers achieved impregnation of a lubricant into a porous protectivecoating formed from thermoplastic resins (Cf. Japanese Unexamined PatentPublication (Kokai) No. 58-200431). The impregnated protective coatinghas excellent resistance to wear, but due to use of thermoplastic resinssuch as phenol, melamine, and epoxy in the formation of the porousprotective coating, shows less CSS strength or mechanical strength.

SUMMARY OF THE INVENTION

According to this invention, there is provided a magnetic recordingmedium including a substrate having formed thereon, in sequence, amagnetic recording layer, a protective coating, and a lubricant coating,the protective coating including a plane portion which occupies asubstantial area of the coating, and a plurality of fine projections andrecesses formed on the plane portion.

In the magnetic recording medium according to this invention, theprojections and/or recesses may be regularly or randomly distributed onthe plane portion of the protective coating. Further, the size andconfiguration of the projections and/or recesses may be the same ordifferent. Further, the projections and/or recesses may be partially orentirely stepped. The projections and recesses may be combined with eachother in the form of craters having different sizes and configurations.The craters may be randomly distributed.

According to this invention, there is also provided a process for theproduction of the above magnetic recording medium including the stepsof: (a) forming a magnetic recording layer on the substrate; (b) coatingthe magnetic recording layer with a protective coating-forming material;(c) selectively etching a layer of the protective coating-formingmaterial; (d) further coating the etched layer of the protectivecoating-forming material with the same material; (e) forming a mask onthe layer of the protective coating-forming material; (f) furthercoating the masked layer of the protective coating-forming material withthe same material; (g) removing the mask and the protectivecoating-forming material overlaid thereon to obtain a protectivecoating; and (h) forming a lubricant coating on the resultant protectivecoating.

In the production process of this invention, the coating step (d) may becancelled and, in place of this step, after the removal step (g) andbefore the coating step (h), the remaining layer of the protectivecoating-forming material may be further coated with the same material toobtain a protective coating. Alternatively, the selective etching step(c) may be repeated two or more times. Further, the coating step (b) andetching step (c) may be repeated two or more times.

According to this invention, there is also provided a process for theproduction of the magnetic recording medium with a plurality of finecraters having different sizes and configurations, including the stepsof: (a) forming a magnetic recording layer on the substrate; (b) coatingthe magnetic recording layer with a solution of a first substancecapable of thermally providing a protective coating-forming material anda second substance capable of forming droplets on the resulting coating,the droplets being thermally decomposed and evaporated; (c) baking theresulting coating having droplets of the second substance to form aprotective coating having distributed thereon a plurality of craters;and (d) forming a lubricant coating on the resultant coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an example of the prior art magneticrecording media;

FIG. 2 is a cross-sectional view of another example of the prior artmagnetic recording media;

FIG. 3A is a cross-sectional view of still another example of the priorart magnetic recording media;

FIG. 3B is a perspective view of the protective coating of FIG. 3A;

FIG. 4A is a cross-sectional view of still another example of the priorart magnetic recording media;

FIG. 4B is a perspective view of the protective coating of FIG. 4A;

FIG. 5A is a cross-sectional view of a preferred embodiment of themagnetic recording medium according to this invention;

FIG. 5B is a perspective view of a surface configuration of therecording medium of FIG. 5A;

FIG. 6A is a cross-sectional view of another preferred embodiment of themagnetic recording medium according to this invention;

FIG. 6B is a perspective view of a surface configuration of therecording medium of FIG. 6A;

FIG. 7A is a cross-sectional view of still another preferred embodimentof the magnetic recording medium according to this invention;

FIG. 7B is a perspective view of a surface configuration of therecording medium of FIG. 7A;

FIGS. 8A and 8B are cross-sectional views of the production steps of therecording medium of FIG. 5A;

FIGS. 9(1) through 9(11) are cross-sectional views of the productionsteps of the magnetic recording medium according to a preferredembodiment of this invention;

FIGS. 10A and 10B are perspective views of a surface configuration ofthe magnetic recording media according to preferred embodiments of thisinvention;

FIGS. 11(1) through 11(18) are cross-sectional views of the productionsteps of the magnetic recording medium according to another preferredembodiment of this invention;

FIG. 12 is a perspective view of a surface configuration of therecording medium produced through the steps of FIGS. 11(1) through11(18); and

FIGS. 13A and 13B show the formation of craters on a silicon dioxideprotective coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 5A shows the magnetic recording medium according to a preferredembodiment of this invention. In the figure, reference numeral 1indicates an aluminum substrate; 2 an alumite coating, which is aproduct of the surface oxidation of the aluminum substrate; 3 a magneticrecording layer, for example, γ-Fe₂ O₃ coating produced throughsputtering or other techniques; and 4 a protective coating, for example,a silicon dioxide coating. Of course, if desired, other light metals andtheir alloys may be used as the substrate in the practice of thisinvention.

The protective coating 4 has a plurality of fine craters 7. As isapparent from FIG. 5B, showing the surface configuration of the magneticrecording medium, the illustrated protective coating 4 includes a planeportion 10, ring-shaped projections 9 and a half sphere-shaped recesses8. Each crater 7 is a combination of the recess 8 and the projection 9.

Another preferred embodiment of this invention can be found in FIGS. 6Aand 6B. As in FIG. 5A, the illustrated recording medium comprises analuminum substrate 1, an alumite coating 2, a γ-Fe₂ O₃ layer 3, and asilicon dioxide protective coating 4. The protective coating 4 has aplane portion 10 having distributed thereon innumerable projections 12and innumerable recesses 11. The projections 12 and the recesses 11 havethe same size and configuration, respectively.

Still another preferred embodiment of this invention can be found inFIGS. 7A and 7B. The layer construction of the illustrated magneticrecording medium is identical with that of FIG. 6A except for theconfiguration of the protective coating. The protective coating 4 hasrecesses 13 having the same configuration and depth, while itsprojections have three different configurations and heights (14a, 14b,and 14c). The projection 14b has a stepped configuration. Of course, therecesses 13 may have different depths and may be stepped, if desired.

As described above, the protective coating of this invention consists ofa plane portion which occupies a substantial area of the coating,innumerable fine projections protruding from the plane portion andinnumerable fine recesses formed in the plane portion. Accordingly, whena lubricant is coated on the protective coating, it fills the recessesand is stably retained therein. As a result, the lubricating action ofthe coated lubricant is maintained and wear of the underlying protectivecoating is prevented for a long period. The minor amount of powder dueto wear will be caught in the recesses and small gaps formed between theadjacent projections. This effectively avoids head crush. Further, sincethe slider of the magnetic head contacts only a top portion of each ofthe projections, close contact of the slider with the protective coatingcan be effectively avoided and the friction coefficient between them canbe remarkably decreased.

The magnetic recording media according to this invention can be producedas follows:

First, production of the magnetic recording medium with fine craterswill be described with reference to FIGS. 8A and 8B. In FIGS. 8A and 8B,illustration of the aluminum substrate is omitted. After the formationof a γ-Fe₂ O₃ magnetic layer 3 on the alumite coating 2, a mixedsolution of a first substance capable of thermally providing aprotective coating-forming material such as silicon dioxide, forexample, Grass Resin™ (Owens Illinois Co.), and a second substancecapable of forming droplets on the resulting coating, the droplets beingthermally decomposed and evaporated, for example, fluid paraffin, in asuitable solvent is spin coated on the magnetic layer 3. The coatingstep may be carried out after a silicon dioxide layer of a thickness ofabout 50 to 300 Å is deposited on the magnetic layer 3 in order tofurther improve the resistance to adhesion of the magnetic head. Afterspin coating, the resultant coating is baked at about 200° C. to 350° C.for about 1 to 5 hours. As a result of the evaporation of droplets ofthe fluid paraffin and the formation of silicon dioxide from Si(OH)₄ inthe Grass Resin, a protective coating consisting essentially of silicondioxide and having innumerable fine craters is formed on the magneticlayer 3.

FIG. 8A is a cross-sectional view of the protective coating-formingmaterial after coating has been completed. As illustrated, droplets 16of the fluid paraffin float in a sea 15 of the Grass Resin. Thesedroplets do not combine to form large droplets due to theirself-repellency. Then, after baking, as is illustrated in FIG. 8B, thefluid paraffin 16 is thermally decomposed and evaporated, while theSi(OH)₄, which is the main component of the Grass Resin, is thermallymodified into silicon dioxide (layer 4). The solvents used for thepreparation of the mixed solution to be coated will be substantiallyevaporated after coating, and the remainder will be completelyevaporated during baking. The resulting silicon dioxide protectivecoating 4 having craters 10 (half-spherical recesses 8 plus ring-shapedprojections 9) is shown in FIG. 8B. Reference number 10 refers to theplane portion of the protective coating.

In the preparation of the illustrated protective coating, any materialcan be used as the first substance insofar as it can provide aprotective coating-forming material such as silicon dioxide afterbaking. Suitable materials include organic silicon compounds such asGrass Resin and other organic metal compounds such as butyllithium,tetraethyl lead, and bis (glyoxymat) nickel. In addition to the fluidparaffin, other materials can be used as the second substance, insofaras they can form droplets floating in the resulting coating and can bedecomposed and evaporated during baking. Suitable materials includeparaffin, vaseline, and naphthalene. These materials should be selectedaccording to the properties of the first substance used therewith. Fluidparaffin is preferably used in an amount of about 0.1 to 2.0% by weightbased on the total weight. Further, many solvents can be used in thepreparation of the mixed solution. Typical examples of these solventsinclude ethylene glycol monoethyl ether acetate, xylene, ethyl acetate,methyl isobutyl ketone, diacetone alcohol, butanol, amyl alcohol, andcyclohexanol. These solvents may be used separately or in combination.

For further understanding of this invention, production of a magneticrecording medium having the cross-sectional view of FIG. 5A will bedescribed in detail hereinafter:

Preparation of the Aluminum Substrate

The disc-shaped aluminum plate is planished with a diamond lathe. Themirror surface of the resulting aluminum substrate is further polishedwith a felt polishing pad to obtain a smooth surface having an Ra of0.01 μm. After polishing, the aluminum substrate is dipped in a sulfuricacid bath to form thereon an alumite coating having a thickness of 3 μm.The alumite coating is also polished to make its surface smoother (Ra=0.006 μm). The polishing is effected with water by pressing on therotating aluminum substrate an alumina tape under high load. After waterpolishing, the substrate is tape burnished and degreased with an alkali.Then, washing is effected, in sequence, with pure water and Freon™ (E.I.du Pont Co.) vapor. After drying, the substrate is baked at 250° C. to300° C. for one hour.

Formation of the Magnetic Layer

Sputtering is effected with an iron alloy target containing iron as itsmain component in an atmosphere of 100% oxygen and at a pressure of1×10⁻² Torr to form a α-Fe₂ O₃ layer having a thickness of 0.2 μm on thealumite coating produced in the previous steps. Then, the α-Fe₂ O₃ layeris subjected to reduction and oxidation processes to modify it to thecorresponding γ-Fe₂ O₃ layer. The reduction process can be carried outat 290° C. for two hours in an atmosphere of moisture-containinghydrogen and the oxidation process for three hours at a dry airatmosphere.

Formation of the Protective Coating

As a pretreatment, sputtering is effected with a silicon dioxide targetin an atmosphere of 100% argon and at a gas pressure of 6.5×10⁻³ Pa toform a silicon dioxide coating having a thickness of 50 to 120 Å on thealumite substrate. Then, the following Grass Resin-containing solutionis spin coated at a speed of 1000 rpm. Composition of the coatingsolution:

    ______________________________________                                        Grass Resin ™ (Si(OH).sub.4 -based silicon                                                          2%                                                   compounds having a Miranol structure,                                         commercially available from Owens                                             Illinois Co.)                                                                 Ethylene glycol monoethyl ether acetate                                                               50%                                                   Xylene                  17%                                                   Ethyl acetate           31%                                                   Fluid paraffin          0.3% of the                                                                   total weight                                          ______________________________________                                    

The coated Grass Resin solution is then baked at 300° C. for one hour inan atmospheric environment. Innumerable craters are formed in theprotective coating.

Coating of the Lubricant

A 0.03% Florinate™ (fluorinated carbon solvent, commercially availablefrom 3M Co.) solution of the lubricant Krytox 143AD™ (perfluoroalkylpolyether, commercially available from E.I. du Pont Co.) is coated onthe silicon dioxide protective coating. The thus obtained magneticrecording medium is photographed with a scanning electron microscope.Electron micrographs FIG. 13A (×1500) and FIG. 13B (×5000) show theformation of the craters on the silicon dioxide protective coating.

The magnetic recording medium of this invention and two typical magneticrecording media according to the prior art are tested to ascertain thecharacteristics of the recording medium according to this invention. Theresults are summarized in the following table.

    ______________________________________                                                 Invention                                                                              Prior art A.sup.1                                                                        Prior art B.sup.2                                ______________________________________                                        Static coefficient                                                                       0.08 to    0.20 to    0.32 to                                      of friction                                                                              0.15       0.80       0.50                                         CSS strength.sup.3                                                                       >50,000    5,000 to   30,000 to                                    (revolutions)         20,000     50,000                                       Resistance to                                                                            >60        <10        30 to 50                                     drag.sup.4 (times)                                                            Resistance to                                                                            >150       <5         >100                                         impact shock.sup.5                                                            (hours)                                                                       Pencil hardness.sup.6                                                                    9H         9H         6H to 8H                                     Adhesion test.sup.7                                                                      Acceptable Acceptable Acceptable                                   ______________________________________                                         Notes:                                                                        Prior art A.sup.1 The test sample comprises an aluminum substrate, a          magnetic Co--Ni--P alloy layer, and a polysilicate protective coating.        Prior art B.sup.2 The test sample comprises an aluminum substrate having      spin coated thereon γ-Fe.sub.2 O.sub.3 containing binders.              CSS strength.sup.3 CSS operation of the magnetic head is repeated to          ascertain the durability of the test samples.                                 Resistance to drag.sup.4 The magnetic head with edges is repeatedly           dragged on the test sample to determine the resistance.                       Resistance to impact shock.sup.5 The positioner of DE is repeatedly           impacted to determine the impact strength.                                    Pencil hardness.sup.6 The surface of the test sample is scratched with th     tips of pencils having different hardnesses. Check the peeling.               Adhesion test.sup.7 Check the change of the static coefficient of frictio     after 24 hours.                                                          

From these results, it is apparent that the magnetic recording mediaaccording to this invention have excellent CSS strength as well assufficient resistance to drag, impact shock, and scratching and do notcause adhesion of the magnetic head to the magnetic layer. Surprisingly,according to this invention, one of the useful lubricants, fluorinatedoil such as Krytox™, can be effectively coated on the protectivecoating, in contrast to the prior art continuous magnetic layers towhich the fluorinated oil is not applicable.

Another preferred process for the production of the magnetic recordingmedium according to this invention will be described with reference toFIGS. 9(1) through 9(11):

1. Sputtering of silicon dioxide (FIG. 9(1))

After the formation of an alumite coating 2 and a magnetic γ-Fe₂ O₃layer 3 on the aluminum substrate (not shown), silicon dioxide isdeposited on the magnetic layer 3 by means of sputtering or othertechniques. Thus, the silicon dioxide layer 4 is formed. Silicon dioxideis particularly advantageous due to its resistance to wear.

2. Formation of the mask (FIG. 9(2))

A coating solution of one can-type polyurethane rubber is coated on thesilicon dioxide layer 4. The coating of polyurethane rubber is thentreated with a solution of Freon™ (fluorohydrocarbons, commerciallyavailable from E.I. du Pont Co.) containing surfactants to change theshape of the rubber. Subsequent baking provides masks 17 of thepolyurethane rubber.

3. Etching (FIG. 9(3))

The silicon dioxide layer 4 is dry etched with an etchant such as carbontetrafluoride (CF₄) gas through the previously produced masks 17. Theexposed area of the silicon dioxide layer 4 is therefore removed.

4. Removal of the masks (FIG. 9(4))

After selective etching in step (3), the used masks 9 are removed bywiping out them with a solvent such as methyl ethyl ketone. Asillustrated in FIG. 9(4), silicon dioxide projections 4 remain on themagnetic layer 3.

5. Sputtering of silicon dioxide (FIG. 9(5))

A new silicon dioxide layer 18 is deposited on the silicon dioxideprojections 4 in a manner similar to that of step (1). The thickness ofthis silicon dioxide layer 18 is preferably about 50 Å.

6. Formation of the mask (FIG. 9(6))

The operation of step (2) is repeated to form masks 19 on the combinedsilicon dioxide layer 4'.

7. Removal of the thin layer of the polyurethane rubber (FIG. 9(7))

In the previous step (6), the thin layer of the polyurethane rubber,which would adversely affect the adhesion of a new silicon dioxide layerto the previously formed silicon dioxide layer 4' in the subsequentstep, is formed around the masks 19. This layer of the polyurethanerubber is removed with dry etching.

8. Sputtering of silicon dioxide (FIG. 9(8))

The operation of step (1) is repeated to form a new silicon dioxidelayer 20. The silicon dioxide layer 20 comprises four portions 20a, 20b,20c, and 20d.

9. Removal of the mask (FIG. 9(9))

The masks 19 formed in step (6) are forcibly wiped off with a suitablewiping solvent such as methyl ethyl ketone. The overlying silicondioxide layers 20b and 20d are also removed.

10. Cleaning (FIG. 9(10))

In order to remove the remaining masks and other undesirable portions,the surface of the combined silicon dioxide layer or protective coating4" is cleaned with a suitable solvent such as methyl ethyl ketone. Asillustrated in FIG. 9(10), the silicon dioxide protective coating 4" hasa plane portion 10, recesses 21a and 21b having different depths, andprojections 22.

11. Coating of the lubricant (FIG. 9(11))

A lubricant such as Krytox™ is coated on the silicon dioxide protectivecoating 4" to form a thin lubricant coating 23. The lubricant isimpregnated into the recesses 21a and 21b and the gaps between theadjacent projections 22. It will be stably retained during the operationof the recording medium. Further, it will be understood that thelubricant on the protective coating is gradually fed to contact portionsof the slider of the magnetic head with the magnetic recording medium.This is very effective for solving the problems of adhesion of themagnetic head and increasing the coefficient of friction and CSSstrength.

In this production process, the sputtering step (5) may be cancelled, ifthe same sputtering operation can be repeated after the cleaning step(10) and before the coating step (11).

The surface configuration of the resulting magnetic medium will varydepending upon the thickness x₁ of the silicon dioxide layer 4 of step(1) and the thickness x₂ of the silicon dioxide layer 20 of step (8).Namely,

(i) In the case of x₁ =x₂, the projections and recesses each have thesame height and depth, as illustrated in FIGS. 6A and 6B.

(ii) In the cases of x₁ >x₂ and x₁ °1 x₂, the projections will havedifferent heights, as in FIGS. 7A and 7B. Further, if x₁ is thicker thanx₂, shallower recesses and higher projections will be formed and,depending upon the masking process, the projections will be stepped. Incontrast, if x₁ is thinner than x₂, deeper recesses and lowerprojections will result.

According to the process of this invention, fine masks having a size onthe submicron order (0.3 to several micrometers) can be easily producedby utilizing the surface tension of certain mask-forming materials suchas one can-type polyurethane rubber (monosan™, commercially availablefrom Indopole Co.). Further, by combining this masking process withetching and lift-off techniques, innumerable fine projections can beproduced with ease. Furthermore, the size and configuration of theprojections and recesses can be controlled depending upon the desiredresults. Distribution of the projections and recesses can also becontrolled. For this control of the projections and recesses, it iscontemplated to suitably select the concentration of the mask-formingmaterials, conditions of the spin coating, conditions of the etching,and the like. For example, stepped projections and recesses will beproduced as a result of change of the etching time. FIGS. 10A and 10Bshow modifications of the surface configuration of the magneticrecording medium according to this invention.

Still another preferred process for the production of the magneticrecording medium according to this invention will be described withreference to FIGS. 11(1) through 11(18):

1. Sputtering of silicon oxide (FIG. 11(1))

Substantially identical with those of FIG. 9(1) except for an increasednumber of silicon dioxide projections 4.

2. Formation of the mask (FIG. 11(2))

Substantially identical with those of FIG. 9(2) except for an increasednumber of silicon dioxide projections 4.

3. Etching (FIG. 11(3))

Substantially identical with those of FIG. 9(3) except for an increasednumber of silicon dioxide projections 4.

4. Removal of the masks (FIG. 11(4))

Substantially identical with those of FIG. 9(4) except for an increasednumber of silicon dioxide projections 4.

5. Formation of the mask (FIG. 11(5))

The operation of step (2) is repeated to form second rubber masks 28.

6. Etching (FIG. 11(6))

The operation of step (3) is repeated except that the etching time wasshortened to one-third of that of step (3). Slightly etched silicondioxide projections 4a are formed.

7. Formation of the mask (FIG. 11(7))

The operation of step (2) is again repeated to form third rubber masks29.

8. Etching (FIG. 11(8))

The operation of step (3) is again repeated to obtain etched silicondioxide projections 4b. The etching time is one-third of that of step(3).

9. Formation of the mask (FIG. 11(9))

The operation of step (2) is again repeated to form fourth rubber masks30.

10. Etching (FIG. 11(10))

The operation of step (3) is again repeated. As illustrated in FIG.11(10), the exposed silicon dioxide projections 4b are completelyremoved.

11. Removal of the masks (FIG. 11(11))

After the completion of etching, the used masks 28, 29, and 30 areremoved by wiping with a solvent such as methyl ethyl ketone. Silicondioxide projections 4, 4a, and 4b remain.

12. Formation of the mask (FIG. 11(12))

The operation of step (2) is again repeated to form masks 31 in adesired pattern.

13. Removal of the thin layer of the polyurethane rubber (FIG. 11(13))

A thin layer of the polyurethane rubber formed around the masks 31 isremoved with dry etching, since it adversely affects the adhesion of anew silicon dioxide layer to the underlying portion in the subsequentstep.

14. Sputtering of silicon dioxide (FIG. 11(14))

The operation of step (1) is repeated to form a new silicon dioxidelayer 32. The silicon dioxide layer 32 comprises seven portions 32a,32b, 32c, 32d, 32e, 32f, and 32g.

15. Removal of the mask (FIG. 11(15))

The masks 31 are forcibly wiped off with a suitable solvent, forexample, methyl ethyl ketone. In addition to these masks 31, theoverlying silicon dioxide layers 32b, 32e, and 32g are also removed.

16. Cleaning (FIG. 11(16))

A surface of the combined silicon dioxide layer 4' is cleaned with asuitable solvent such as methyl ethyl ketone. As is shown, the silicondioxide layer 4' has a plane portion 10, recesses 33a 33b and 33c havingdifferent depths, and projections 34a, 34b, and 34c having differentheights.

17. Sputtering of silicon dioxide (FIG. 11(17))

An additional silicon dioxide layer 35 is formed on the cleaned silicondioxide layer 4' in a manner similar to that of step (1).

18. Coating of the lubricant (FIG. 11(18))

A lubricant such as Krytox™ is coated on the resulting silicon dioxideprotective coating 4" to form a thin lubricant coating 36. The lubricantis stably retained in the recesses 33a, 33b, and 33c, and the gapsformed between the adjacent projections 34a, 34b, and 34c.

The surface configuration of the resulting protective coating isillustrated in FIG. 12. Projections 38a, 38b, and 38c have the heights300 Å, 200 Å, and 100 Å, respectively. The depth of the recesses 37 is500 Å. Each projection and recess has a diameter of about 2 to 3 μm.

We claim:
 1. A magnetic recording medium comprising a substrate havingformed thereon, in sequence, a magnetic recording layer, a protectivecoating, and a lubricant coating, said protective coating being onematerial and comprising a plane portion which occupies a substantialarea of the coating, and a plurality of fine projections and recessesformed on said plane portion.
 2. A magnetic recording medium as in claim1, in which said projections and/or recesses are regularly distributedon said plane portion.
 3. A magnetic recording medium as in claim 1, inwhich said projections and/or recesses are randomly distributed on saidplane portion.
 4. A magnetic recording medium as in any of claims 1through 3, in which said projections and/or recesses have the same sizeand configuration.
 5. A magnetic recording medium as in any of claims 1through 3, in which said projections and/or recesses have differentsizes and configuration.
 6. A magnetic recording medium as in claim 1,in which said projections and/or recesses are partially or entirelystepped.
 7. A magnetic recording medium as in claim 1, in which saidprojections and recesses are combined with each other and are in theform of craters having different sizes and configurations.
 8. A magneticrecording medium as in claim 7, in which said craters are randomlydistributed on said plane portion.
 9. A magnetic recording medium as inclaim 1, wherein said projections are friction-reducing projections andsaid recesses are lubricant-retaining recesses.
 10. A magnetic recordingmedium comprising a substrate having formed thereon, in sequence, amagnetic recording layer, a protective coating means for reducingfriction and retaining lubricant, and a lubricant coating, saidprotective coating means being made of one material and comprising aplane portion which occupies a substantial area of the coating, and aplurality of fine friction-reducing productions and lubricant-retainingrecesses formed on said plane portion.